1. Field of the Invention
The present invention relates in general to an apparatus and method for building of a workpiece by deposit welding. In particular, it relates to a reusable preform for use in the shape melting welding process.
2. Description of the Related Art
Shape melting is a process whereby structural components are manufactured by depositing weld material layer-upon-layer until the desired geometry is achieved. This process offers the advantage of delivering a near-net-shape product which can be produced quickly and economically using both ferrous and non-ferrous materials.
U.S. Pat. No. 2,299,747 to Harter is directed in part to a method for forming structures wholly of fusion deposited weld metal by the arc process in successive intersecting fusion beads along a non-adherent base. A similar method is described in U.S. Pat. No. 2,912,562 to Donovan which disclosure is directed to reconditioning cylinder liners for diesel engines. The concept of forming a cylinder made up solely of welded material progressively deposited in molten form is disclosed in U.S. Pat. No. 3,558,856 to Ujiie. Million, et al, U.S. Pat. No. 4,517,434 relates to deposit welding to make a tube bend build up by a plurality of weld sections. Additionally known from U.S. Pat. No. 4,621,762 to Bronowski is the buildup of a workpiece by deposit welding using form shoes cooled by water. Along the same general lines as Ujiie, U.S. Pat. No. 4,671,448 to Million, et al describes a method of forming an element having a symmetrically curved surface by means of weld buildup and rotation of the work.
Automation and computer control can be integral parts of the shape melting process. The use of automation allows for maximum flexibility of the production station. This flexibility permits any number of different products to be manufactured without extensive retooling. Retooling in this case would be, for the most part, changing the controlling software which dictates the sequence, welding parameters, and position of welds necessary to achieve the desired final product.
Almost every facet of shape melting can involve computer control. In the incipient states of a project, design aspects would be subject to computer analysis. The results of these analyses would then be incorporated into the functions which control automation.
Implicit in the use of the above controls is the ability to deliver a final product with tailored mechanical, corrosion, and physical properties. This comes about because these properties are strongly tied to the interrelated functions of weld heat input, cooling rate, bead size, bead shape, bead sequence, and bead position. If, in addition to the above variables, one also permits controlled composition variation througout the product, it is possible, if appropriate control is exercised, for the product to have the desired combination of strength, toughness, hardness, or corrosion resistance at critical points in the product.
Weld build-up operations like shape melting require a conventional preform which is generally a machined piece of metal onto which the first layer of the build-up is deposited. It is termed a "preform" because its formed or machined shape reflects an intended final shape of the build-up.
A conventional preform serves as the support for the molten as-deposited weld metal, as the conduit for conduction cooling of the freshly deposited weld metal, as the means for restraining weld contraction stresses thereby limiting distortion of the build-up, and as the general cross-sectional shape for the weld build-up, e.g. a cylindrical build-up would require a cylinder as the starting preform.
Virtually all weld build-ups require some form of preform for any or all of the purposes stated above. In almost every instance, the surface of the conventional preform is melted by the heat of the welding arc. This melting of the surface results in a detrimental bonding of the conventional preform to the weld build-up. Further, unless the conventional preform has the same composition as the weld filler material, surface melting of the conventional preform will result in the initial layers of the build-up having a composition which includes some melted conventional preform material.
If the aforementioned composition variation is objectionable, it will be necessary to machine away the conventional preform and as many layers of the build-up as necessary to achieve an acceptable weld metal composition throughout. This loss of material and increased production time negatively impacts the economy of shape melting. As was mentioned earlier, another purpose associated with the use of a conventional preform, is that the conventional preform usually must be machined to an initial desired geometry. This implies expenses both in materials and machining time prior to shape melting. Thus, in summary, if the need for a conventional preform can be eliminated, the costs associated with both the initial and final stages of shape melting manufacturing can be reduced.